The present invention is directed to a continuous process for making water-insoluble, crosslinked, high molecular weight polymers capable of absorbing and retaining large quantities of aqueous fluids (SAP). These polymers are well known in the art by various names such as superabsorbent polymers, hydrogels, hydrocolloids, and water absorbent hydrophilic polymers, etc. For the purpose of this description the term, "SAP" means superabsorbent polymer, collectively referring to such materials.
Exemplary superabsorbent polymers include crosslinked, partially neutralized polyacrylic acid (see U.S. Pat. No. 4,654,039); crosslinked, partially neutralized starch-acrylic acid graft polymer (U.S. Pat. No. 4,076,663); crosslinked, partially neutralized copolymer of isobutylene and maleic anhydride (U.S. Pat. No. 4,389,513); saponification product of vinyl acetate-acrylic acid copolymer (U.S. Pat. No. 4,124,748); hydrolyzate of acrylamide polymer or acrylamide copolymer (U.S. Pat. No. 3,959,569); or hydrolyzate of an acrylonitrile copolymer (U.S. Pat. No. 3,935,099). The teachings of the above patents as embodiments of superabsorbent materials are hereby incorporated by reference.
Superabsorbent polymers find use in many fluid absorption applications with the primary use being in the field of personal care products such as diapers, sanitary napkins, adult incontinent products, absorption pads for medical uses, etc. The largest market for superabsorbent polymers is found in disposable diapers for infants; see e.g. U.S. Pat. Nos. 3,669,103; 3,670,731 or 4,654,039.
Numerous other patents disclose superabsorbent polymers and their uses, such as U.S. Pat. No. 4,076,663; 4,552,938; 4,507,438; 4,5235,098; 4,820,773 and European Patent Application 189,163.
U.S. Pat. No. 4,985,518 discloses a method of preparing a solid water absorbing resin including mixing a monomer solution of (A) acrylic acid neutralized 70-100 mole percent; and (B) a water-miscible to water-soluble polyvinyl monomer in a combined concentration of up to about 50 wt. %; with water to form a mixed monomer solution, and adding a thermal initiator and a redox initiator to the mixed monomer solution to form an initiated mixed monomer solution at a temperature below the decomposition temperature of the thermal initiator. The addition of the redox initiator thereto and polymerization of the monomers causes a rise in the temperature of the initiated mixed monomer solution to a level sufficient to activate the thermal initiator.
U.S. Pat. No. 4,857,610 assigned to Stockhausen, discloses a moving conveyor apparatus for the continuous production of SAP and copolymers of water-soluble monomers particularly acrylic acid and/or methacrylic acid at up to about 40% monomer solids. The trough-like shape of the conveyor belt changes continuously into an extended flat profile during the polymerization process; starting from the side edges and working towards the center of the trough formed by the conveyor belt. The resulting polymer gel strand is released continuously during the transition of the curved trough-like shape of the conveyor belt into the extended, flat form.
U.S. Pat. No. 4,625,001 assigned to Nippon Shokubai discloses a continuous process to make SAP comprising the steps of continuously feeding an aqueous solution of a SAP forming monomer in aqueous solution to effect polymerization into a water-containing cross-linked polymer along with a polymerization initiator. The vessel is provided with a plurality of mutually parallel rotary stirring shafts each fitted with stirring blades. The process includes finely dividing a water-containing gel polymer issuing from the polymerization in progress by the shearing force of stirring blades generated by the rotation of said stirring shafts while allowing the radical aqueous solution polymerization to proceed without interruption, and continuously discharging the resultant finely divided water-containing gel polymer out of said vessel.
Among the known batch processes for making SAP, there is the aqueous solution batch process at relatively low monomer solids (about 30 solids) coupled to drum-type dryers; there is the aqueous solution batch process coupled to a tunnel-type dryer. The aforementioned processes represent a significant capital cost per unit of SAP produced.
A continuous process has been discovered for use with a variety of reaction vessel shapes which avoids polymerization on a moving conveyor, and the need for agitation equipment thereby providing greater simplicity and ease of operation with relatively low capital cost per unit of SAP produced.